160 Isokontae 



Order 1. PROTOCOCCALES. 



The Algae of this order are mostly unicellular, motile or non-motile, or less 

 frequently simple coanocytes. The cells are often aggregated to form loose 

 irregular colonies and are frequently embedded in a copious mucilage. In a few 

 forms there is a definite coenobium 1 , which may be a coanobium of coenocytes, 

 but a definite multicellular thallus does not occur. The order includes the 

 lowest and the most primitive of the Green Algse, forms through which 

 all the higher groups of the Isokontse have been evolved. Various members 

 of the Protococcales are to be found in every conceivable situation in which 

 Algae will grow, and they display a wonderful diversity of character. The 

 cell-wall is sometimes very delicate, but at other times firm and thick, and 

 there is often a great development of mucilage. The number of chloroplasts 

 varies much in the different genera, and so does their disposition. Pyrenoids 

 may or may not be present. 



Many of the lower types multiply by simple cell-division in one or more 

 directions of space. All the various reproductive processes found within 

 the whole range of the Isokonta? occur in the Protococcales. One type of 

 asexual spore (or gonidium), the autospore, is worthy of special mention. 

 A number of autospores are generally produced within the distended mother- 

 cell, each spore usually developing the complete (or almost complete) characters 

 of the parent-cell before liberation (figs. 121, 125, 126, etc.). In some of the 

 coenobic forms the plants are reproduced by the formation of autocolonies 

 (figs. 128 and 133). 



The three lines of descent which are generally recognized as originating in the Proto- 

 coccales necessitate a subdivision of the group into three series as follows : 



Sub-order Volvocinese. 



Family Polyblepharidaceae. 

 Family Sphaerellacese. 

 Family Volvocacese. 



Sub-order Tetrasporinese. 



Family Palmellacese. 

 Family Dictyosphaeriacese. 

 Family Protococcacese. 

 Family Autosporacese. 

 Family Chsetopeltidacese. 



Sub-order Chlorococcineae. 



Family Planosporacese. 

 Family Hydrodictyacese. 



1 A ccenobiuin is a compact colony of definite form, the number of cells in which remains 

 constant so long as the colony exists. All the cells arise either as gonidia or zoogonidia within 

 the original mother-cell, and therefore belong to the same generation. The adult ccenobium is 

 formed by the regular grouping of the motile or non-motile gonidia, no further increase in the 

 number of cells ever taking place. 



